High efficiency passively Q-switched mode-locking Nd:GdVO4 laser with LT-In0.25Ga0.75As saturable absorber

The generation of passively Q-switched mode-locking operation with 100% modulation depth has been observed from a diode-pumped Nd GdVO4 laser with a low temperature In0.25Ga0.75As saturable absorber, which was grown by the metal-organic chemical-vapor deposition technique and acted as saturable absorber as well as output coupler. The repetition rate and pulse duration of the mode-locked pulses concentrated in the Q-switch envelop were 455 MHz and 12 ps, respectively. The average output power was 1.8 W and the slope efficiency was 36%. (C) 2009 Elsevier B.V. All rights reserved.

Optical properties of aluminum-, gallium-, and indium-doped Bi4Ti3O12 thin films

Undoped and Al-, Ga-, and In-doped Bi4Ti3O12 thin films were prepared on fused quartz substrates by chemical solution deposition. Their microstructures and optical properties were investigated by x-ray diffraction and UV-visible-NIR spectrophotometer, respectively. The optical band-gap energies, Urbach energies, and linear refractive indices of all the films are derived from the transmittance spectrum. Following the single oscillator model, the dispersion parameters such as the average oscillator energy (E-0) and dispersion energy (E-d) are achieved. The energy band gap and refractive indices are found to decrease with introducing the dopants of Al, Ga, and In, which is useful for the band-gap engineering and optical waveguide devices. The refractive index dispersion parameter (E-0/S-0) increases and the chemical bonding quantity (beta) decreases in all the films compared with those of bulk. It is supposed to be caused by the nanosize grains in films. (c) 2009 American Institute of Physics. [DOI 10.1063/1.3138813]

Equilateral-Triangle and Square Resonator Semiconductor Microlasers

The characteristics of equilateral-triangle resonator (ETR) and square resonator microlasers are reported, which are potential light sources in the photonic integrations. Based on the numerical simulations, we find that high-efficiency directional emission can be achieved for the triangle and square microlasers by directly connecting an output waveguide to the resonators. The electrically injected InP/InGaAsP ETR and square resonator microlasers with a 2-mu m-wide output waveguide were fabricated by standard photolithography and inductively coupled plasma etching techniques. Room-temperature continuous-wave (CW) operations were achieved for the ETR microlasers with the side length from 10 to 30 mu m and the square resonator microlasers with the side length of 20 mu m. The output power versus CW injection current and the laser spectra are presented for an ETR microlaser up to 310 K and a square resonator microlaser to 305 K. The lasing spectra with mode wavelength intervals as that of whispering-gallery-type modes and Fabry-Perot modes are observed for two square lasers, which can lase at low temperature and room temperature, respectively.

Electrically Pumped Room-Temperature Pulsed InGaAsP-Si Hybrid Lasers Based on Metal Bonding

A pulsed InGaAsP-Si hybrid laser is fabricated using metal bonding. A novel structure in which the optical coupling and metal bonding areas are transversely separated is employed to integrate the silicon waveguide with an InGaAsP multi-quantum well distributed feedback structure. When electrically pumped at room temperature, the laser operates with a threshold current density of 2.9 kA/cm(2) and a slope efficiency of 0.02 W/A. The 1542 nm laser output exits mainly from the Si waveguide.

High-performance evanescently-coupled uni-traveling-carrier photodiodes

A new evanescently-coupled uni-traveling-carrier photodiode (EC-UTC PD) based on a multimode diluted waveguide (MDW) structure is fabricated, analysed and characterized. Optical and electrical characteristics of the device are investigated. The excellent characteristics are demonstrated such as a responsivity of 0.36 A/W, a bandwidth of 11.5 GHz and a small-signal 1-dB compression current greater than 18 mA at 10 GHz. The saturation current is significantly improved compared with those of similar evanescently-coupled pin photodiodes. The radio frequency (RF) bandwidth can be further improved by eliminating RF losses induced by the cables, the probe and the bias tee between the photodiode and the spectrum analyzer.

Fabry-Perot microcavity modes observed in the micro-photoluminescence spectra of the single nanowire with InGaAs/GaAs heterostructure

We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111) B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Perot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Perot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field. (C) 2009 Optical Society of America

Reduced divergence angle of photonic crystal vertical-cavity surface-emitting laser

The reduced divergence angle of the photonic crystal vertical-cavity surface-emitting laser (PC-VCSEL) was investigated in both theory and experiment. The photonic crystal waveguide possessed the weakly guiding waveguide characteristic, which accounted for the reduction of the divergence angle. The three-dimensional finite-difference time-domain method was used to simulate the designed PC-VCSEL, and a calculated divergence angle of 5.2 degrees was obtained. The measured divergence angles of our fabricated PC-VCSEL were between 5.1 degrees and 5.5 degrees over the entire drive current range, consistent with the numerical results. This is the lowest divergence angle of the fabricated PC-VCSEL ever reported.

1310 nm GaInNAs triple quantum well laser with 13 GHz modulation bandwidth

The emission wavelength of a GaInNAs quantum well (QW) laser was adjusted to 1310 nm, the zero dispersion wavelength of optical fibre, by an appropriate choice of QW composition and thickness and N concentration in the barriers. A triple QW design was employed to enable the use of a short cavity with a small photon lifetime while having sufficient differential gain for a large modulation bandwidth. High speed, ridge waveguide lasers fabricated from high quality material grown by molecular beam epitaxy exhibited a damped modulation response with a bandwidth of 13 GHz.

Chirp-free optical modulation using a silicon push-pull coupling microring

We propose a silicon ring-based optical modulation method to perform chirp-free optical modulations. In this scheme, we locate the light to be modulated at the resonance of the ring and tune the coupling coefficient between the ring and the straight waveguide by using a push-pull coupling structure. The chirp-free phase modulation can be achieved by varying the coupling coefficient in a large range, which can modify the coupling condition of the ring such that the input light experiences an abrupt phase shift of pi at the output. If the coupling coefficient is adjusted in a small range such that the coupling condition of the ring is kept unchanged, only the intensity of the light will be modulated. This leads to chirp-free intensity modulation. Our simulations performed at 10 Gbits/s confirm the feasibility of the proposal. (C) 2009 Optical Society of America

40-Gb/s Low Chirp Electroabsorption Modulator Integrated With DFB Laser

A 40-Gb/s monolithically integrated transmitter containing an InGaAsP multiple-quantum-well electroabsorption modulator (EAM) with lumped electrode and a distributed-feedback semiconductor laser is demonstrated. Superior characteristics are exhibited for the device, such as low threshold current of 20 mA, over 40-dB sidemode suppression ratio at 1550 nm, and more than 30-dB dc extinction ratio when coupled into a single-mode fiber. By adopting a deep ridge waveguide and planar electrode structures combined with buried benzocyclobutene, the capacitance of the EAM is reduced to 0.18 pF and the small-signal modulation bandwidth exceeds 33 GHz. Negative chirp operation is also realized when the bias voltage is beyond 1.6 V.

Experimental Demonstration on Structure-Parameter Dependence of Photonic Crystal Optical Spectrum

We present fabrication and experimental measurement of a series of photonic crystal waveguides and coupled structure of PC waveguide and PC micro-cavity. The complete devices consist of an injector taper down from 3 mu m into a triangular-lattice air-holes single-line-defect waveguide. We fabricated these devices on a silicon-on-insulator substrate and characterized them using tunable laser source. We've obtained high-efficiency light propagation and broad flat spectrum response of photonic-crystal waveguides. A sharp attenuation at photonic crystal waveguide mode edge was observed for most structures. The edge of guided band is shifted about 31 nm with the 10 nm increase of lattice constant. Mode resonance was observed in coupled structure. Our experimental results indicate that the optical spectra of photonic crystal are very sensitive to structure parameters.

Confinement factor and absorption loss of AlInGaN based laser diodes emitting from ultraviolet to green

Confinement factor and absorption loss of AlInGaN based multiquantum well laser diodes (LDs) were investigated by numerical simulation based on a two-dimensional waveguide model. The simulation results indicate that an increased ridge height of the waveguide structure can enhance the lateral optical confinement and reduce the threshold current. For 405 nm violet LDs, the effects of p-AlGaN cladding layer composition and thickness on confinement factor and absorption loss were analyzed. The experimental results are in good agreement with the simulation analysis. Compared to violet LD, the confinement factors of 450 nm blue LD and 530 nm green LD were much lower. Using InGaN as waveguide layers that has higher refractive index than GaN will effectively enhance the optical confinement for blue and green LDs. The LDs based on nonpolar substrate allow for thick well layers and will increase the confinement factor several times. Furthermore, the confinement factor is less sensitive to alloys composition of waveguide and cladding layers, being an advantage especially important for ultraviolet and green LDs.

Metamorphic InGaAs Quantum Well Laser Diodes at 1.5 mu on GaAs Grown by Molecular Beam Epitaxy

We report a 1.5-mu m InGaAs/GaAs quantum well laser diode grown by molecular beam epitaxy on InGaAs metamorphic buffers. At 150 K, for a 1500 x 10 mu m(2) ridge waveguide laser, the lasing wavelength is centred at 1.508 mu m and the threshold current density is 667 A/cm(2) under pulsed operation. The pulsed lasers can operate up to 286 K.

An investigation of the mode characteristics of SOI submicron rib waveguides using the film mode matching method

The mode characteristics of SOI (silicon-on-insulator) submicron rib waveguides are very different from those of micrometer-sized ones. Using the full-vector film mode matching method, we propose a simple criterion to determine whether a waveguide mode is guided or not. The single-mode condition for deep-etched waveguides is obtained using this criterion. We also obtain the inherent TM mode leakage and sharp cancelation effects due to TE-TM mode coupling in shallow-etched rib waveguides from numerical simulations, which agree well with the analytical results based on total internal reflection and interference theories.

Investigation of mode coupling in a microdisk resonator for realizing directional emission

Mode coupling between the whispering-gallery modes (WGMs) is numerically investigated for a two-dimensional microdisk resonator with an output waveguide. The equilateral-polygonal shaped mode patterns can be constructed by mode coupling in the microdisk, and the coupled modes can still keep high quality factors (Q factors). For a microdisk with a diameter of 4.5 mu m and a refractive index of 3.2 connected to a 0.6-mu m-wide output waveguide, the coupled mode at the wavelength of 1490 nm has a Q factor in the order of 10(4), which is ten times larger than those of the uncoupled WGMs, and the output efficiency defined as the ratio of the energy flux confined in the output waveguide to the total radiation energy flux is about 0.65. The mode coupling can be used to realize high efficiency directional-emission microdisk lasers. (C) 2009 Optical Society of America